Prior to this commit, the build instructions were encoded in Makefiles
that contained GNU extensions that are unique to GNU Make and
incompatible with older implementations.
Thie commit renames all Makefiles to GNUmakefile which clearly denotes
that the file contains GNU extensions.
GNU Make actually first looks for a GNUmakefile, then makefile, THEN
Makefile so in addition to simply being more correct and portable, it
saves a few unnecessary failed attempts to open the correct build
file.
Prior to this commit, our Makefiles assumed the name of the make program
was simply "make".
Since we use GNU make extensions, this isn't always the case --
operating systems like FreeBSD come with their own implementation named
make which can be incompatible with the extensions used in by tinygo.
To run a compatible make on some of these systems, we have explicitly
call GNU make by running gmake.
This commit changes references to the command "make" to "$(MAKE)" which
is a variable that contains the name of the executable invoked to
process the Makefile.
This allow the Makefiles to be uniformly processed by the same make
program.
